Freshly drawn optical fibre in the pristine state, free from surface defects, is capable of exhibiting strengths in excess of 10.sup.6 psi. However, this must be regarded as ultimate strength rather than that routinely attainable over long lengths. A major difficulty in achieving very high strength fibres has been the inability to rigorously limit the size or completly eliminate the presence of surface flaws on the fibre. Surface flaws may originate from a number of sources. They may exist prior to and persist after drawing as is the case of residual contaminants and defects from the support tube used for the preform preparation. Damage may also be induced in the surface of the preform during processing and handling. Other sources include the presence of devitrification at the surface of the preform and contact with any foreign substance during and after drawing, including particulate debris and condensates from furnace refactories, heating elements and dust particles in the drawing environment. Misalignment of coating applicators and foreign particles in coating materials are also potential sources of weak fibre.
In order to keep the fibre surface as defect free as possible it is essential to apply a protective coating on-line. However, pre-existing flaws on the fibre surface act as stress concentrators and, in the presence of moisture, these flaws slowly grow in size until the local stress concentration is sufficiently high that catastrophic growth begins and results in fibre fracture. Thus, the fibre breaks at service stresses far below the initial fibre strength. This process of strength degradation with time as a result of crack enlargement by the combined action of stress and moisture is known as Stress Corrosion or Static Fatigue. Therefore, in order to guarantee a certain lifetime for a fibre one must proof test at a level much higher than it would be expected to see in service and thus allow for strength degradation by stress corrosion. Alternatively, one prevents or drastically reduces stress corrosion such that proof test strains can be comparable with those actually expected during service i.e. the strength of the fibre will remain constant with time.
For stress corrosion to occur two conditions must prevail:
(i) stress at a surface flaw and PA0 (ii) moisture must be available at this flaw.
Under these conditions nucleophilic attack by water can cause disruption of Si--0--Si linkages. In order to prevent the occurrence of stress corrosion one must simply remove one or both of the requirements indicated above. Installed fibres are normally under a small but significant stress. This, coupled with higher stresses experienced during installation or recovery of a cable, means that it is not feasible for the fibre to experience zero stress during all or most of its projected lifetime (25 years for underwater telecommunications systems). Consequently, the only retaining means of inhibiting stress corrosion is to prevent moisture reaching the fibre surface. While organic coatings, either thermally or UV cured, provide excellent abrasion resistance, ambient moisture will quickly penetrate the coating hence allowing stress corrosion to proceed.
For example thermally cured silicones are easy to apply and cure but provide a poor water barrier and present curing problems at high pulling speed. Ultra-violet cured polymers are also easy to apply and they cure rapidly, but still do not provide a satisfactory water barrier.
Hermetic coatings can be obtained by metallization, freeze coating from melt-amorphous metals and by ceramic coatings such as silicon nitride or silicon oxynitride. Metallization coatings have been shown to weaken the fibre although they provide a very effective water barrier; freeze coating from melt-amorphous metals also provides a very effective water barrier and is elastic to about two percent strain, but need very high quench ratios for high speed pulling; ceramic coatings are highly adherent and provide a very effective water barrier but can involve the use of toxic materials and elevated diposition temperatures, making them very difficult to handle for on line coatings.